[tip: perf/urgent] perf: Fix event leak upon exit

["tip-bot2 for Frederic Weisbecker" <tip-bot2@xxxxxxxxxxxxx>]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     73caafba7021a97c22bff58c3d123228e03cdc46
Gitweb:        https://git.kernel.org/tip/73caafba7021a97c22bff58c3d123228e03cdc46
Author:        Frederic Weisbecker <frederic@xxxxxxxxxx>
AuthorDate:    Fri, 21 Jun 2024 11:16:00 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 25 Jun 2024 10:43:38 +02:00

perf: Fix event leak upon exit

When a task is scheduled out, pending sigtrap deliveries are deferred
to the target task upon resume to userspace via task_work.

However failures while adding an event's callback to the task_work
engine are ignored. And since the last call for events exit happen
after task work is eventually closed, there is a small window during
which pending sigtrap can be queued though ignored, leaking the event
refcount addition such as in the following scenario:

    TASK A
    -----

    do_exit()
       exit_task_work(tsk);

       <IRQ>
       perf_event_overflow()
          event->pending_sigtrap = pending_id;
          irq_work_queue(&event->pending_irq);
       </IRQ>
    =========> PREEMPTION: TASK A -> TASK B
       event_sched_out()
          event->pending_sigtrap = 0;
          atomic_long_inc_not_zero(&event->refcount)
          // FAILS: task work has exited
          task_work_add(&event->pending_task)
       [...]
       <IRQ WORK>
       perf_pending_irq()
          // early return: event->oncpu = -1
       </IRQ WORK>
       [...]
    =========> TASK B -> TASK A
       perf_event_exit_task(tsk)
          perf_event_exit_event()
             free_event()
                WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1)
                // leak event due to unexpected refcount == 2

As a result the event is never released while the task exits.

Fix this with appropriate task_work_add()'s error handling.

Fixes: 517e6a301f34 ("perf: Fix perf_pending_task() UaF")
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240621091601.18227-4-frederic@xxxxxxxxxx
---
 kernel/events/core.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8f908f0..7c3218d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2284,18 +2284,15 @@ event_sched_out(struct perf_event *event, struct perf_event_context *ctx)
 	}
 
 	if (event->pending_sigtrap) {
-		bool dec = true;
-
 		event->pending_sigtrap = 0;
 		if (state != PERF_EVENT_STATE_OFF &&
-		    !event->pending_work) {
-			event->pending_work = 1;
-			dec = false;
+		    !event->pending_work &&
+		    !task_work_add(current, &event->pending_task, TWA_RESUME)) {
 			WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
-			task_work_add(current, &event->pending_task, TWA_RESUME);
-		}
-		if (dec)
+			event->pending_work = 1;
+		} else {
 			local_dec(&event->ctx->nr_pending);
+		}
 	}
 
 	perf_event_set_state(event, state);